1. Field of the Invention
The present invention relates to a high frequency signal switching unit for switching a plurality of output terminals to selectively output therefrom a high frequency signal inputted from an input terminal or for switching between at least one input terminal and at least one output terminal.
2. Description of the Related Art
FIG. 17 depicts a conventional high frequency signal switching unit (known as the coaxial switch or high frequency unit) that includes three high frequency relays 10 fixed within a generally box-shaped metallic casing 1 by means of a conductive adhesive. The high frequency relays 10 are connected at their relay contact terminals 11 to five coaxial connectors 3 mounted on a side wall of the metallic casing 1 by coaxial cables 30, and also connected at their coil terminals 12 to a relay connector 40 mounted on another side wall of the metallic casing 1 by lead wires 41. The central coaxial connector 3 is an input terminal into which a high frequency signal is inputted, while other four coaxial connectors 3 are output terminals from which the high frequency signal is selectively outputted.
Each high frequency relay 10 is provided with a shielding plate 4 disposed around the relay contact terminals 11. Each coaxial cable 30 includes an internal conductor (core) 31 connected to one of the relay contact terminals 11 by soldering, and also includes an external conductor (in the case of the illustrated coaxial cable 30, the whole sheath is the external conductor) 32 joined to the shielding plate 4, which is in turn grounded. The shielding plate 4 is provided with chip capacitors C mounted thereon for absorbing high frequency signals that leak from the coaxial cables 30 to the lead wires 41.
By the above-described construction, the high frequency relays 10 are supplied with electricity via the relay connector 40 so that the high frequency signal inputted from one of the coaxial connectors 3 forming the input terminal may be selectively outputted from any one of the remaining four coaxial connectors 3.
In the conventional high frequency signal switching unit of the above-described construction, because the high frequency relays 10 are fixed to the metallic casing 1 by means of the conductive material, the positions of the high frequency relays 10 are not always fixed. Furthermore, because the coaxial cables 30 are supported by only the opposite connecting portions, a difficulty is encountered in positioning and temporarily fixing the coaxial cables 30 during assembling, thus lowering the working efficiency and increasing variation in the product characteristics. Also, shielding is not performed between the relay contact terminals 11 of the neighboring high frequency relays 10, thus deteriorating the insulation properties (high frequency characteristics).
The present invention has been developed to overcome the above-described disadvantages.
It is accordingly an objective of the present invention to provide a high frequency signal switching unit having improved high frequency characteristics, wherein the positions of the high frequency relays are fixed, and the coaxial cables can be readily positioned and temporarily fixed.
In accomplishing the above and other objectives, the high frequency signal switching unit according to the present invention includes a casing and a printed circuit board accommodated in the casing and having a first surface and a second surface opposite to each other. The printed circuit board also has a ground layer formed on substantially the entire first surface. The high frequency signal switching unit also includes at least one input terminal and at least one output terminal both mounted on the casing, and at least one high frequency relay mounted on the printed circuit board for switching between the input terminal and the output terminal so that a high frequency signal inputted into the input terminal is selectively outputted from the output terminal. The high frequency relay has a plurality of relay contact terminals and a grounding terminal. A plurality of coaxial cables having respective internal and external conductors are accommodated in the casing to connect the input and output terminals to the relay contact terminals of the high frequency relay. The grounding terminal of the high frequency relay and the external conductors of the coaxial cables are connected to the ground layer of the printed circuit board, while each of the internal conductors of the coaxial cables is connected to one of the relay contact terminals of the high frequency relay.
With this arrangement, because the high frequency relay is secured to the printed circuit board, the position thereof is fixed. Accordingly, the positioning and temporary fixing of the coaxial cables within the casing can be readily carried out. Furthermore, the grounding terminal of the high frequency relay and the external conductors of the coaxial cables are all connected to the ground layer of the printed circuit board, making it possible to improve the high frequency characteristics.
The printed circuit board has a plurality of recesses defined therein in which end portions of the coaxial cables are respectively received. This arrangement further facilitates the positioning of the coaxial cables relative to the printed circuit board.
Preferably, the external conductors have a diameter substantially equal to a width of the recesses. With this configuration, when the coaxial cables are inserted into respective recesses, the former are temporarily fixed. Accordingly, the distance between the relay contact terminals of the high frequency relay and the internal conductors of the associated coaxial cables is made substantially constant, thus stabilizing the high frequency characteristics.
The recesses are elongated in the axial directions of the coaxial cables. By so doing, the temporary fixing of the coaxial cables relative to the printed circuit board can be readily carried out, making it possible to enhance the working efficiency during assemblage of the coaxial cables.
The depth of the recesses is determined such that the internal conductors are aligned with the second surface of the printed circuit board. By so doing, the distance between the relay contact terminals of the high frequency relay and the internal conductors of the associated coaxial cables can be reduced, enhancing the high frequency characteristics.
The printed circuit board has a plurality of grooves defined therein so as to extend across a thickness thereof, in which end portions of the coaxial cables are respectively received. Because the formation of the grooves in the printed circuit board is relatively easy, the manufacturing cost of the high frequency signal switching unit can be reduced.
Advantageously, each of the grooves has an electrically conductive layer formed on an inner wall thereof, which is connected to the ground layer of the printed circuit board. The formation of the electrically conductive layer increases the contact area and connecting strength of the coaxial cables. Furthermore, the external conductors of the coaxial cables and the ground layer of the printed circuit board are connected at a reduced distance, resulting in an increase in ground strength and ensuring more reliable grounding.
Again advantageously, the printed circuit board has a plurality of through-holes defined therein, each of which communicates with a neighboring one of the grooves to accommodate one of the relay contact terminals of the high frequency relay. Each of the through-holes has an electrically conductive layer formed on an inner wall thereof, which is connected to the ground layer of the printed circuit board. The electrically conductive layer acts to shield the relay contact terminals, making it possible to enhance the insulation properties.
Conveniently, each of the internal conductors has a planar free end and a through-hole defined therein, into which one of the relay contact terminals is inserted. This configuration facilitates the assembling work and enhances the connecting strength between the internal conductors and the relay contact terminals.
The printed circuit board may be of a multi-layered construction having two layers formed one upon another with strip lines made of an electrically conductive material interposed therebetween. The strip lines are intended to connect the relay contact terminals of the high frequency relay to the internal conductors of the coaxial cables. In this case, the printed circuit board also has two ground layers formed on external surfaces of the two layers, respectively. By this construction, the positions of the coaxial cables and those of the relay contact terminals relative to the printed circuit board can be changed while suppressing the insertion loss as small as possible. Accordingly, the lengths of the coaxial cables can be made substantially constant, making it possible to reduce the manufacturing cost thereof.
The high frequency signal switching unit further includes a relay connector mounted on the casing for supplying the high frequency relay with electricity, wherein the high frequency relay has coil terminals, and the printed circuit board has a wiring pattern formed in the ground layer for connecting the relay connector to the coil terminals. The formation of the wiring pattern can reduce the number of the component parts and the size of the high frequency signal switching unit, compared with the case wherein the relay connector and the coil terminals are connected via lead wires.
Advantageously, the printed circuit board has a plurality of grooves defined therein so as to extend across a thickness thereof, in which end portions of the coaxial cables are respectively received. Each of the grooves has an electrically conductive layer formed on an inner wall thereof, which is connected to the two ground layers of the printed circuit board. With this arrangement, the two ground layers of the printed circuit board and the external conductors of the coaxial cables are all connected together via the electrically conductive layers, making it possible to further improve the high frequency characteristics.
It is preferred that the printed circuit board be made of a fluorine-contained resin. Because the fluorine-contained resin has a small dielectric dissipation factor, not only can the printed circuit board be made thin, but the insertion loss can also be reduced.
Advantageously, the printed circuit board has a plurality of recesses defined therein on one side thereof where connecting portions between the internal conductors of the coaxial cables and the strip lines are exposed. With this arrangement, the internal conductors of the coaxial cables can be connected directly to the strip lines and, hence, the insertion loss can be reduced. Also, the shielding between the neighboring strip lines can be achieved.
The high frequency signal switching unit further includes a conductor connected to the two ground layers of the printed circuit board, wherein the recesses of the printed circuit board are closed by the conductor. The conductor acts to enhance the shielding properties.
The conductor may be a ground layer formed on substantially one entire surface of another printed circuit board and connected to the two ground layers of the printed circuit board. The ground layer acts to enhance the shielding properties. In this case, if a component part adjacent to the strip lines is an insulator (substrate), short-circuiting can be prevented.
The high frequency signal switching further includes a relay connector mounted on the casing for supplying the high frequency relay with electricity, wherein the high frequency relay has coil terminals, and the another printed circuit board has a wiring pattern formed in the ground layer for connecting the relay connector to the coil terminals. With this arrangement, the possibility of high frequency signals jumping from the strip lines of the multi-layered printed circuit board to the wiring pattern for connecting the relay connector and the coil terminals of the high frequency relay is reduced. As a result, capacitors that have been hitherto used to absorb leakage of the high frequency signals can be dispensed with, making it possible to reduce the number of the component parts and the manufacturing cost.